There has so far been known as a brake system for a vehicle (a car), for example, a brake system including a booster device such as a negative pressure booster and a hydraulic pressure booster. In recent years, an electric booster device (an electric brake actuator) using an electric motor as a boosting source has been known (for example, see Patent Document 1).
The electric booster device disclosed in Patent Document 1 is configured to include a main piston which is reciprocated by an operation of a brake pedal (a brake operation element), a cylindrical booster piston which is fitted externally to the main piston so as to be displaceable relative to the main piston, and an electric motor which reciprocates the booster piston.
The electric booster device has the main piston and the booster piston as pistons of a master cylinder and have each of front end portions of the pistons facing a pressure chamber of the master cylinder, and is capable of generating a brake hydraulic pressure in the master cylinder, by a thrust force inputted to the main piston from the brake pedal by the operator and by a booster thrust force inputted to the booster piston from the electric motor. In a brake system including an electric booster device, the brake system including a stroke simulator, which allows a stroke of the brake pedal by absorbing a brake fluid sent out by the master cylinder, has been also known (for example, see Patent Document 2).